1. Field of the Invention
The present invention relates to photocatalyst granules capable of decomposing an organic matter by employing light energy, and a method of preparing the same.
The present application is based on Japanese Patent Application No. Hei 11-371947 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
The photocatalytic reaction is a reaction that decomposes an organic matter by using a photocatalyst in the presence of light energy. Since this reaction belongs to a technique of decomposing harmful organic matter by employing ultraviolet light included in sunlight or indoor illumination, such as fluorescent lamp or incandescent lamp, its application for purification of water or air and construction materials such as building materials and tiles has widely been studied.
As the photocatalyst, titanium dioxide has generally been used. The organic matter is decomposed and made innoxious by being irradiated ultraviolet light in the presence of titanium dioxide. As the raw material of titanium dioxide photocatalyst, fine particles of titanium dioxide and an alkoxide solution of metallic titanium have conventionally been employed.
When using fine titanium dioxide particles, there can be used a method of using fine titanium dioxide particles as they are, a method of using fine titanium dioxide particles after fixing them on a solid using a binder, and a method of using titanium dioxide particles after mixing them with a coating composition or a hydrate, followed by coating on the solid, drying and further fixation. There can be used what is termed a sol-gel method, wherein the alkoxide solution of metallic titanium is used as the raw material. After coating a solid with this solution, drying the solution and firing the dried matter to form a titanium dioxide thin film, this film is used as the photocatalyst.
There can also be used a method of using as a photocatalyst after forming a titanium dioxide thin film on a solid using a CVD or sputtering method.
When using the titanium dioxide fine particles as they are, the photoactivity is high and the adsorbability of the organic matter is high because of the large surface area. However, it is hard to handle because of the fine particles, thereby making it difficult to control the position of the fine particles when using them in an air or liquid. In case these fine particles are scattered or dispersed, these scattered or dispersed fine particles cause scattering of ultraviolet light, resulting in poor photoefficiency. It is also difficult to recover the fine particles.
When using the fine particles after they are fixed on a solid using a binder, the fine particles are coated with or embedded in the binder. Therefore, the surface area of the fine particles in contact with the organic matter, oxygen or ultraviolet light is lowered, thereby lowering the decomposition efficiency of the organic matter. When using an organic binder, there arises a problem that binder itself is decomposed by the titanium dioxide fine particles.
When used after forming a thin film from an alkoxide solution of metallic titanium on a solid using a sol-gel method, there is a merit that the thin film does not exert any influence on the form of the solid and a thin film can be formed between small spaces. An anatase type crystal of titanium dioxide having high photocatalytic activity can be formed by firing at a temperature of 600° C. or less. However, it takes a long time to prepare a thin film of the anatase type crystal and, moreover, the preparation conditions cannot be controlled easily. There is also the problem that the contact area with the organic matter is smaller than that in the case of the fine particles of titanium dioxide because of the thin film, and thus the decomposition efficiency is low and the adsorbability of the organic matter is poor. There is also the problem that the durability of the thin film is low.